The present invention relates to communications systems and the like and, more particularly, to the improvement for permitting closer spacing of the communications from the nodes on the path of a communications system wherein a reference node and at least one slave node transmit over a common path on a time sharing basis via a common media to provide 100% utilization of that media, comprising, means for causing the reference node to transmit a unique signal at the start of a transmission sequence; means for the slave node to monitor the transmission from the reference node and for adjusting the bit rate, phasing, framing and sequencing of the slave node until the reference node and slave node are placed in precise synchronization; means for informing the reference node that the slave node is in synchronization and for beginning normal transmission from the reference node; and, means for causing the slave mode to begin its normal transmission immediately following the end of the normal transmission from the reference node without a delay therebetween.
In the discussion of the prior art and the present invention contained hereinafter, a satellite communications system is used as the point of reference and example. It is to be understood by those skilled in the art, however, that the invention described and claimed herein will work with and is applicable to any communications network including, but not limited to, computer networks, fiber optic communication systems, etc. Similarly, while a satellite communication system employing a satellite as a central hub or distribution point is described herein, the invention can be used with various communications network configurations well known in the art such as loop, star, hybrid, etc.
Communications systems are being used in ever-increasing popularity as more and more communications networks are placed into service. A typical satellite communications system is shown in simplified form in FIG. 1. There are a plurality of ground stations 10 which, for ease of identification, are labelled as "A" through "G". The ground stations all communicate through a single satellite 12. Each ground station 10 has an antenna 14 aimed at the satellite 12 and the satellite 12 has one or more antennas 16 each aimed towards the ground 18 and covering an area or footprint 20 on the ground 18 from which that antenna can receive transmissions and to which it can transmit. Communication via the satellite 12 is on a time sharing or time slicing basis. Thus, insofar as the present inventions and the benefits achievable thereby are applicable to other time slicing and time sharing environments and apparatus, it is the applicant's intent that they be included within the scope and spirit of this application, the specification, and claims thereof.
To say that a satellite for communications is expensive would be a gross understatement. Between the cost of the satellite itself and the cost of launching it into space by rocket, the cost of a single communications satellite in place and functioning is in the millions of dollars. This is all to say that, once in place, a prime goal is to maximize the utilization of the satellite. Any available time that is not used in the communications process represents wasted time and money.
The prior art time sharing use of communications satellites such as satellite 12 of FIG. 1 is depicted in the timing diagram of FIG. 2 taken in conjunction with the overall placement of the ground stations 10 and satellite 12 shown in FIG. 1. As any ground station 10 transmits to the satellite 12, the satellite 12 rebroadcasts what is being received back towards the footprint 20 on ground 18 from from one of its antennas 16. Thus, all ground stations 10 within the footprint 20 can receive the transmissions from any other ground station 10. It should be noted that a transmitting ground station 10 can receive its own transmission via the satellite 12 as well. Thus, for example, in the limited system of FIG. 1, when ground station "B" is transmitting, the retransmittion thereof from the satellite 12 can be received by ground stations "A", "B", "C", and "D". The same would be true with respect to stations "E", "F", and "G". While there is also transmission between footprints via the satellite 12, that is not of concern here.
In a prior art satellite communications system such as that of FIG. 1, all the ground stations 10 within a common footprint 20 are aware of the distance (in time) of each other station 10 from the satellite 12. Thus, any ground station 10 can predict the time that a transmission from any other station 10 will take to reach the satellite 12, be retransmitted, and reach the receiving station. This information is used to time slice the available time between the ground stations 10 as depicted in the diagram of FIG. 2. Each ground station 10 is allocated a fixed time duration for its transmissions within each overall time subdivision. For example, if the time subdivision of the satellite 12 were one second, each ground station 10 might be allocated the same fixed number of milliseconds in which to do its transmitting within each second. The actual times involved are much faster and the above numbers are just employed for purposes of the example. The order of transmission is fixed and each actual transmission includes a header block identifying the originating station 10 and the station or stations 10 for which the transmission is intended. This is very much like most computer-based time sharing and multiplexing schemes work. In this manner, each station 10 can identify the sequence of transmissions as received from the other stations 10 via the satellite 12. Since each station 10 knows the distance in time for the other stations 10 it can calculate the time at which it should begin and end its transmissions so as to fit properly within its allocated space in the time sequence.
As depicted in the time sequence of FIG. 2, however, the timing coordination of the prior art satellite communications systems is by approximation. There is a reference station 10 (for example station "A") which controls each transmission sequence. Each sequence is begun by the reference station and the remaining stations calculate their time slot after acquisition and lock-on to the originating signal from the reference station. As a result, in order to assure that each station 10 does not "step on" or transmit on top of the ending portion of the transmission from the station immediately preceeding it in the time sequence, a slight delay or buffer is allowed between the end of one station's transmission and the beginning of the transmission from the next. Thus, in FIG. 2 we see a transmission 22 from station "A" followed by a delay 24. Then, there is a transmission 22 followed by another delay 24, and so forth. While these delays 24 are of short duration, they still represent a series of lost times within each time sequence. The more individual stations 10 are transmitting within each time sequence, the more individual delays 24 there are and the more total time is lost to delays. Additionally, the acquisition sequence must be repeated for each transmission sequence.
The INTELSAT TDMA/DSI System is the most commonly known prior art example of the environment wherein the present invention is applicable. By way of providing a complete background with respect to the prior art in edited form, the following description is included herein. It is a condensed version of that contained in the "Reference Manual for Telecommunications Engineering" and is, in turn, extracted from Pontano, Dicks, et al., "The INTELSAT TDMA/DSI System" from INTELSAT, Washington, DC.
The earth segment of the INTELSAT/DSI system comprises four reference stations per satellite and a number of traffic terminals. The system operates with satellites having four coverage areas (east hemispheric beam, west hemispheric beam, east zone beam and west zone beam). Normally, zone beam coverage areas will also be contained within hemispheric beam coverage areas. Zone and hemispheric beams use opposite senses of polarization. FIG. 3 shows a satellite with typical east-to-west and west-to-east connectivities of both the zone and hemispheric beams. Two dual polarized reference stations located in each zone coverage area are thus able to monitor and control both zone and hemispheric beam transponders. Each reference station generates one reference burst per transponder and each transponder will be served by two reference stations. This provides redundancy by enabling traffic terminals to operate with either reference burst. The two pairs of reference stations provide network timing and control the operation of traffic terminals and other reference stations.
Reference stations include a TDMA system monitor (TSM) which is used to monitor system performance and diagnose system faults. In addition, the TSM is used to assist users in carrying out their traffic terminal lineups.
The traffic terminals operate under control of a reference station and transmit and receive bursts containing traffic and system management information. Traffic terminals contain interfaces which are used to connect the terminals to the terrestrial networks. The TDMA/DSI system uses two types of interfaces: digital speech interpolation (DSI), which accommodates voice traffic (together with a limited amount of nonvoice traffic), and digital noninterpolated (DNI), which accommodates data and noninterpolated voice traffic.